Twig one's lay

: Twig one's lay. I've gathered this means "understand one's position / approach / scheme," and I found the pieces in wiktionary (twig: 'From Irish and Scots Gaelic tuig, "to understand"'; lay: "Arrangement or relationship; layout"), but it'd be nice to read some history on this. Is it a common phrase, or just an invention used in the kind of books I read?

It's not only genuine, it's extremely old. In 18th-century English thieves' cant (slang), "twig" meant "watch, observe", and therefore "grasp the meaning of"; it's still used in everyday British slang. (It *may* come from Gaelic; but it is first recorded in 1764, long before there had been significant Irish immigration into England, so how likely is it that English low-life would have been using irish words? The jury is still out on that one.) "Lay" in thieves' cant meant "ploy, occupation, line of business" - e.g. lock-picking was known as "the dub lay", and stealing brass weights from shop counters was "the avoirdupois lay". (VSD)